Recently, there is increasing interest in the power generation using natural energy such as solar power generation or wind-power generation from the viewpoint of resource protection and prevention of global warming. In consideration of this, there is developed a power supply system capable of supplying buildings with the power generated by the sunlight in addition to or in place of the power from an electrical grid (see Patent Literature 1 below).
However, the electric power generated by using the natural energy (hereinafter also referred to as “generated electric power”) largely varies with a change in the amount of solar radiation or a wind velocity. The electric power demand in a building largely varies with daily time period. Typically, peak daily time periods of these differ from each other.
The power supply system generally includes an electric storage apparatus in order to temporarily store the generated electric power in excess of the demand in the building and to supplement the generated electric power insufficient for the demand by discharging. For example, the electric storage apparatus is available as a stationary storage battery and may be available as a storage battery that is included in an electric vehicle and is cable-connected to the building. Efficiently charging and discharging the electric storage apparatus for every daily time period in consideration of demand-and-supply balance of the electric power (balance between an electric power demand in the building and the generated electric power) can suppress the electric power supplied from the electrical grid to the building and reduce the electric power rate to be paid to an electric power company.
Patent Literature 1 discloses a power supply system that can optimize a charge-discharge schedule for the electric storage apparatus based on a transition of the power generation quantity and the future electric power demand predicted.
The electric storage apparatus can store a limited amount of electric power. Suppose the generated electric power remains large for a long time. In such a case, reverse power flow is needed to supply not-stored surplus electricity back to the electrical grid. The reverse power flow is favorable from the viewpoint of reducing an electric power rate by electric power selling. Patent Literature 1 also discloses a method of optimizing a charge-discharge schedule on condition that the electric power selling minimizes the electric power rate.
However, an excessively increasing number of buildings performing the reverse power flow causes an undesirable decrease in the quality of electric power (causing frequencies to be unstable) in the electrical grid. It is therefore considered impractical to widely use a power supply system that presupposes the reverse power flow. In consideration of this, it is more favorable to minimize an occurrence of reverse power flow and use the generated electric power for a large percentage of electric power demand in a building, namely, promote local production for local consumption in terms of electric power.